Re: Automotive EMI influence on a MEMS IMU, what can I do ?

"None really" <serpent17@xxxxxxxxx> wrote in message
news:1120427333.437284.156630@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> Larry Brasfield wrote:
>> "None really" <serpent17@xxxxxxxxx> wrote in message
>> news:1120424736.353363.241540@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
>> > Hello,
>> Hi.
>> > I am using an IMU made of MEMS that has the particularity to provide
>> > not only acceleration, rate of rotation but also heading.
>> > The heading is a mangetic one. When I use this unit in my car, I have
>> > noise in readings at two levels. First, it looks like I have DC
>> > component that, I guess, could be taken care of through the use of a
>> > calibration scale (like they do in small airplanes). Second, I get a
>> > high frequency component that makes it very annoying to even think I
>> > can use this for a good heading. The second part can somehow be
>> > attenuated through a gyro compensated input, and it works well when the
>> > car is running (not driving). However, when the car is driven, even the
>> >
>> > gyro compensated heading is not accurate (high frequency noise).
>>
>> High frequency noise does not imply inaccuracy.
>
> I agree with you, however, the heading being constant within reason
> (car is driven in straight line on a nice road at low speed 5 mph),
> the shift in the heading reading is much larger than what would be
> expected by taking into account
> the vibration induced by the car frame onto the IMU. I wish it were
> just that.
>
>>
>> > Moving
>> >
>> > the IMU to the back of the car helps when the car is not moving (but
>> > the engine is on), however it does not help when the car is driven.
>> > Even when you put the IMU on top of the car outside, the same noise is
>> > still there, making it difficult for the IMU to be used for the heading
>> >
>> > information.
>> >
>> >
>> > So I am thinking I would like to use a faraday cage, is this a good
>> > idea ? Should I put the cage around the engine or the IMU (they are of
>> > different sizes) ? What type of materials and grid should I use ?
>>
>> I doubt you have to bother with that.
>>
>> > when I looked at this thread, I seem to recognize my problem:
>> >
>> > http://groups-beta.google.com/group/sci.electronics.design/browse_frm/thread/c8bf8ef1adb22bc0/da0b55313eb5bfe4?q=emi+car&rnum=1&hl=en#da0b55313eb5bfe4
>>
>> There I see only something similar to your conclusion
>> that EMI is the problem. Nothing you have stated,
>> beyond your obvious assumption, makes me think so.
>
> well I guess I am making this statement because of the following:
> - when the car engine is not running, the IMU gives the right heading.
> - when the engine is running but the car is not moving, moving the IMU
> away from the dashboard (like in the back seat) make the high frequency
>
> reading of the IMU heading go back to normal.
> - when the car is driven in straight line, the same high frequency
> "noise"
> seems to come back in full force with amplitude beyond what would be
> expected from usual high frequency noise as stated above. In other
> words, the heading
> information is just absolutely not reliable.

That conclusion is not supported by the evidence given.

>> > so I looked for materials for Automotive EMI and found the following:
>> > http://www.lairdtech.de/ENGLISH/products.htm
>> >
>> > which one would be useful ?
>> >
>> > Any pointers would be very much appreciated. Thanks in advance,
>>
>> Have you evaluated the signal(s) in question after applying
>> adequate low-pass filtering to suppress that noise and avoid
>> aliasing at the ADC? If not, that may be all you need. If so,
>> why are you concerned about the high frequency noise?
>>
>
> One of the ways to do low-pass filtering is to couple the heading
> information with the
> acceleration and rate of rotation of the car.

I do not understand that statement. My interpretation
is that you have not taken steps to avoid aliasing.

> It does absolutely great
> when the car is not driving
> and the engine is running and the IMU is in the back seat. However,
> those same type of high frequency
> readings come back in full force (with amplitude beyond my expectation)
> when the car is driven on a nice road at low speed. Even an average of
> this last signal does not even out to the mean heading expected.

If aliasing has already occured, averaging may
not be enough to straighten it out even if there
was nothing other than a high frequency signal
added to an uncorrupted low frequency signal.

>From what you have stated, it is possible that
high frequency noise is getting into the device
circuits through the power supply or through
space (EM coupling). It would certainly be
interesting to see what the HF noise looks like
if you add filtering to the supply and put the
device into a shielded enclosure. If those have
any effect on the average value of the signal,
evaluated with appropriate anti-aliasing for any
sampled signal, then that would indicate that
the noise you worry about is, in fact, a problem.

But at this point, I see no reason to believe you
have anything but additive noise that simply has
not been appropriately filtered prior to sampling.
Can you provide evidence to the contrary?

--
--Larry Brasfield
email: donotspam_larry_brasfield@xxxxxxxxxxx
Above views may belong only to me.


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